The present invention relates to a hydraulic power booster, in particular for the actuation of master cylinders in brake systems of automotive vehicles. More particularly, the invention relates to a hydraulic booster of the type including a pressure fluid source, a valve assembly, by means of which hydraulic pressure is adapted to be introduced into a booster chamber in dependence upon an actuating force, and an axially slidable stepped booster piston which confines the booster chamber and which includes two interconnected piston parts of different diameters which are movable axially relative to one another by a predetermined amount. One such hydraulic booster is disclosed in the German patent application P No. 35 26 475.6.
A similar power booster is disclosed in German publication No. 33 15 730.8. In that power booster, the brake pedal-side end surface of the booster piston includes an extension terminating in an enlarged head. The head or extension is supported with axial play in an annular piston which is part of a piston rod that is connected with a brake pedal. The annular piston includes an axially outwardly extending annular collar which takes support on a shoulder of the housing in the brake's release position. When an actuating force is exerted on the brake pedal, the booster chamber is first isolated from an unpressurized supply reservoir by a brake valve. As the actuating force is increased, displacement of a control slide of the brake valve allows pressure fluid to flow out of a pressure source into the booster chamber, whereby the booster piston shifts and displaces in relation to the annular piston. At a certain pressure level in the booster chamber, the booster piston, after having overcome friction, will move together with the master cylinder piston in the actuating direction so that hydraulic pressure develops in the working chamber of the master cylinder. At a second predetermined pressure in the booster chamber of the hydraulic power booster the booster piston will have displaced in relation to the annular piston an amount equal to the axial play of the booster so that the head of the booster piston moves into abutment on the mating surface of the annular piston. This arrangement results in a relatively high level of noise and bouncing of the two pistons on one another.